The present invention relates to a high power, fast repetition rate and high performance magnetically delayed vacuum switch.
It is not easy to switch on a high-voltage, high-current electrical circuit. For example, where a charged capacitor is to be discharged through a load, it is necessary to have a way of isolating the capacitor from the load during the charging time, and permitting current to flow from the capacitor to the load when discharge is desired.
If the capacitor is to be charged to high potential, and if the amount of energy to be transmitted to the load is substantial, common mechanical and solid-state switches may not be used. Some common switching devices cannot withstand the high voltage across the switch before it conducts. Others cannot carry the high current that flows through the switch when it conducts. Still others may not begin conducting at a predictable time.
In some applications it is desirable to operate cyclically, with a repeated charging of the capacitor and triggering of the switch to dissipate the energy in the load. Yet some switches capable of withstanding the voltage and current levels are incapable of recovering quickly enough to permit repeated triggering at the desired rate.
One device used for such switching is the well-known vacuum-insulated switching gap, also called a vacuum switch. In a vacuum switch, two main electrodes are arranged so as to be separated by a vacuum gap. Current flows, if at all, from one terminal of the charged capacitor to one main electrode, across the gap to the other main electrode, to one terminal of the load. A return path connects the other terminal of the load to the other terminal of the capacitor. Until the switch is triggered, no substantialy current flows.
Within the vacuum envelope and in addition to the two main electrodes, a trigger electrode is also provided. To permit current to flow across the gap between the main electrodes, a trigger current is passed between the trigger electrode and one of the main electrodes. The trigger current creates a plasma, which provides current carriers in the region between the main electrodes. Current then flows between the main electrodes, and the energy previously stored in the capacitor is delivered to and dissipated in the load. After the capacitor has discharged, the plasma disperses, and the switch no longer conducts. If desired, the capacitor may be recharged again for a subsequent discharge.
The vacuum switch enjoys a high voltage standoff; very little or no current flows prior to the moment the switch is turned on, even with a very high voltage across the switch. It also has a wide triggering range, so that for any of a range of voltages across the switch, it is capable of being turned on when triggered to do so. It has fast recovery: after one pulse has passed through the switch, the switch quickly becomes non-conductive. This permits a high repetition rate of charge/discharge cycles.
A vacuum switch, however, has drawbacks. For example, the switch has poor turn-on performance. An ideal switch would instantly begin conducting, and immediately carry all the desired current; a graph of current as a function of time would show a step. However, state-of-the-art vacuum switches turn on rather raggedly. A graph of current as a function of time would show a ramp or even a more irregular pattern, prior to the moment when maximum current is reached.
A vacuum switch may also suffer from jitter, that is, a fluctuation, over a series of charge/discharge cycles, in the delay between the moment of triggering and the moment when full current flow is achieved.
A vacuum switch may also suffer from high power dissipation. Prior to triggering, no power is dissipated in the switch because no current flows. After full conduction occurs, the voltage drop across the switch is minimal, so that power dissipation within the switch is minimal. But during the transition time from non-conduction to full conduction, a substantial voltage drop may exist across the switch at a time when substantial current is flowing, giving rise to unwanted dissipation of power within the switch.
These shortcomings are a direct result of the non-zero interval, sometimes called channel-preparation time or vapor traversal period, required to fill the gap between the main electrodes with a low pressure metallic vapor of sufficient density to support a high discharge current.
In an effort to minimize the difficulties caused by these shortcomings of the vacuum switch and of many other types of switch, it has been attempted in the prior art to use a pulse compression system with the switch. In a pulse compression system, a number of saturable ferromagnetic elements are separated by intervening energy stores so as to compress the pulse provided to the load. The pulse compression system, however, has the drawback that intermediate energy stores are required and makes for greater system volume.
It is an object of this invention to provide a switch which enjoys the advantages of a vacuum switch (high voltage standoff, wide triggering range, very fast recovery, and a high repetition rate) and yet does not have the disadvantages of poor turn-on performance, large jitter, and high power dissipation.
It is yet a further object of the invention to provide a switch with improved characteristics, without the necessity of the greater system volume and intermediate energy stores of a pulse compression system.
It is another object of the invention to provide a high performance switch with a fast repetition rate, and the ability to control high power.